A video posted on Reddit shows an impressive McCree ultimate just after a Reinhardt Earthshatter to get a full team kill. It's especially impressive because only three members of the enemy team actually got hit by the Earthshatter and the McCree had to time his ultimate well because there were two Zarya bubbles at the beginning.
The video shows a Reinhardt player hammer down half of the enemy team right before McCree uses his ultimate to kill all of them. Both ultimates are fairly short so making sure that the McCree is in position and the two ultimates can line up can be difficult. The fact that there are two Zarya bubbles at the start of the ultimate and the McCree shoots just as they go down just adds another layer of difficulty to this feat.
Overwatch is unique because of its flashy ultimate abilities like Reinhardt's Earthshatter and McCree's Deadeye. But on their own they aren't enough to take out the enemy team. Coordination and combos like this one are the best way to win a teamfight, in this case, by a landslide.